1. Field of the Invention
The present invention relates to a digital time base corrector having a drop-out compensating function in an apparatus for reproducing a video signal recorded on a recording medium.
2. Description of Background Information
When a video signal recorded on a video disc is simply demodulated and reproduced, fine jitters are included in the reproduced video signal due to an eccentric component of the video disc and the vibration of a mechanism system. This results in the time base error which causes a deviation of the reproduced video signal from a reference time base. As means for correcting such a time base error, there is known a digital time base corrector such that after the reproduced video signal has been converted into digital data by an A/D converter, it is written into a memory, and the digital data is sequentially read out synchronously with a reference timing signal in accordance with the writing order and is returned to the analog signal by a D/A converter.
As a conventional digital time base corrector, there is a corrector disclosed in Japanese Patent Kokai No. 1-93273. In such a time base corrector, there is provided a phase locked loop (PLL) circuit such that at least one of a horizontal sync signal and a color burst signal is separated and extracted from the reproduced video signal and the separated signal is supplied to the PLL circuit. The PLL circuit forms a clock signal whose phase is synchronized with a time base error included in the reproduced video signal, and the clock signal is phase-modulated by phase modulating means in accordance with a phase comparison output of phase comparing means in the PLL circuit. The clock signal after completion of the phase modulation is used as a sample timing signal of the A/D converter. The clock signal, consequently, also traces the high frequency component of the time base error and the correction is performed.
On the other hand, in such a digital time base corrector, there is a digital time base corrector having a drop-out compensating function. For example, as disclosed in Japanese Utility Model Kokai No. 3-125577, a drop-out detecting circuit to detect a drop-out of a reproduction video signal is provided, and when a drop-out is detected, the writing operation into a memory is inhibited, and digital video data which has already been stored in the memory is read out in such an inhibition operating period of time. That is, the memory is used as a 1 H delay element.
In such a conventional digital time base corrector having the drop-out compensating function, a memory capacity of the memory is set to a capacity of only 1 H (H: horizontal scan period of time) of the video signal. In case of the video signal of the NTSC system, however, since the phase of the chrominance signal is inverted by 180.degree. for every 1 H, there is a problem such that a chrominance signal processing circuit must be separately provided in order to keep the interleave relation between the luminance signal and the chrominance signal of the video signal.